This invention relates to methods for manufacturing silanes having silanol groups by using organoalkoxy silanes as a starting material.
Linear organopolysiloxanes having hydroxyl groups at both molecular ends are useful as dispersing agents in the manufacturing of silicone rubber compounds, and various kinds of dispersing agents have been employed. The effectiveness of the dispersing agents is proportional to the content of the hydroxyl groups. Therefore, a polysiloxane having more hydroxyl groups in its content, that is, a low molecular weight linear organosiloxane having silanol groups as its terminal groups can be used in a lesser amount and is thus more effective from the processability viewpoint of the silicone rubber compounds. However, linear organopolysiloxanes currently employed on an industrial level are organotetrasiloxanes or larger. There have been hardly any proposals to obtain more effective lower molecular weight organosilanes or siloxanes having silanol groups, such as monomer diols, dimer diols, and trimer diols, on an industrial level having the indicted advantages.
Conventionally, much research has been carried out on synthesis methods for short-chained hydroxy silanes and siloxanes. In the laboratory, a known method is to hydrolyze alkoxy silanes while maintaining the solution neutral by employing, for example, a buffer solution. However, this method is difficult to perform on an industrial level. A further method includes refluxing dimethoxy silane which is mixed with neutral distilled water in excess, however, the yield of this reaction is not very good.
Low molecular weight linear polyorganosiloxanes having terminal silanol groups may be industrially manufactured by the hydrolysis of linear organochlorosiloxanes or chlorosilanes having chlorine atoms at both molecular ends in a weakly alkaline aqueous solution in order to prevent them from forming cyclic compounds. However, in this method, HCI produced by the hydrolysis may precede the condensation reaction, since silanol groups are unstable against acids and alkalis. As a result, higher molecular weight organopolysiloxanes and cyclic polysiloxanes or silanes are also produced in addition to the intended organopolysiloxanes or silanes. Therefore, this synthesis method is not particularly useful unless there is a means to maintain the hydrolysis water strictly neutral, which is difficult.
Another method known is the acetoxylation of the organochloropolysiloxanes with acetic acid, followed by hydrolysis. However, this method does not complete the hydrolysis fully and the residual acetoxyl groups remain in the product. Therefore, the product is not desirable as a dispersing agent for silicone rubber.
On the other hand, U.S. Pat. No. 3,925,285 discloses a synthetic method for low molecular weight linear polyorganosiloxanes having silanol terminal groups along with a small amount of residual methoxyl groups through the reaction of hexamethyl cyclotrisiloxane, methanol, formic acid, and water. This method is rather costly due to the employment of the relatively expensive hexamethyl cyclotrisiloxane. In addition, it will not form low molecular weight linear polyorganosiloxanes having silanol terminal groups with less than three D units [(CH.sub.3).sub.2 SiO]. Further, compounds shorter than 1,5-dihydroxy trisiloxane cannot be formed from this reaction. Thus, the content of hydroxyl groups is limited.
Further, U.S. Pat. No. 5,057,620 discloses a method wherein the corresponding chlorosiloxanes are added drop by drop to the epoxy type solvents containing water, such as propylene oxide and butylene oxide. Again in this method, the relatively expensive hexamethyl cyclotrisiloxane is utilized. In addition, there is a safety problem of electrostatic ignition when using a low boiling point solvent.
Further, Japanese Patent Application, Kokoku (examined) 64-5604 describes a synthesis method for short-chained silanols wherein alkoxy silanes are hydrolyzed by solid acid catalysts such as activated clay. In this method, it is necessary to neutralize the solid acid catalysts, which makes the process complicated. In addition, this method results in a mixture of various kinds of short-chained silanols and the yield for the dimer diol is about 50%. The yield of the monomer diol is at most 10% with 84% purity, as seen in Comparison Example 3 below.
U.S. Pat. No. 3,304,318 shows a method of manufacturing resins with a high degree of polymerization through the hydrolysis of alkoxy silanes by utilizing cation-exchange resins. However, this patent does not mention any application to the synthesis of short-chained silanols, particularly the synthesis of dimer diols or their yields.
The present invention was made, with a view to these deficiencies of the prior art, with the object of providing manufacturing methods for organosilanes having silanol groups, particularly diorganodihydroxy silanes, at a high yield, with a simple process, and at a low cost.